.\" $OpenBSD: EVP_PKEY_CTX_set_hkdf_md.3,v 1.4 2024/07/10 07:57:37 tb Exp $
.\" full merge up to: OpenSSL 1cb7eff4 Sep 10 13:56:40 2019 +0100
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.\" This file was written by Alessandro Ghedini <alessandro@ghedini.me>,
.\" Matt Caswell <matt@openssl.org>, and Viktor Dukhovni <viktor@dukhovni.org>.
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.Dd $Mdocdate: July 10 2024 $
.Dt EVP_PKEY_CTX_SET_HKDF_MD 3
.Os
.Sh NAME
.Nm EVP_PKEY_CTX_set_hkdf_md ,
.Nm EVP_PKEY_CTX_set1_hkdf_salt ,
.Nm EVP_PKEY_CTX_set1_hkdf_key ,
.Nm EVP_PKEY_CTX_add1_hkdf_info ,
.Nm EVP_PKEY_CTX_hkdf_mode
.Nd HMAC-based Extract-and-Expand key derivation algorithm
.Sh SYNOPSIS
.In openssl/evp.h
.In openssl/kdf.h
.Ft int
.Fo EVP_PKEY_CTX_hkdf_mode
.Fa "EVP_PKEY_CTX *pctx"
.Fa "int mode"
.Fc
.Ft int
.Fo EVP_PKEY_CTX_set_hkdf_md
.Fa "EVP_PKEY_CTX *pctx"
.Fa "const EVP_MD *md"
.Fc
.Ft int
.Fo EVP_PKEY_CTX_set1_hkdf_salt
.Fa "EVP_PKEY_CTX *pctx"
.Fa "unsigned char *salt"
.Fa "int saltlen"
.Fc
.Ft int
.Fo EVP_PKEY_CTX_set1_hkdf_key
.Fa "EVP_PKEY_CTX *pctx"
.Fa "unsigned char *key"
.Fa "int keylen"
.Fc
.Ft int
.Fo EVP_PKEY_CTX_add1_hkdf_info
.Fa "EVP_PKEY_CTX *pctx"
.Fa "unsigned char *info"
.Fa "int infolen"
.Fc
.Sh DESCRIPTION
The
.Dv EVP_PKEY_HKDF
algorithm implements the HKDF key derivation function.
HKDF follows the "extract-then-expand" paradigm, where the KDF logically
consists of two modules.
The first stage takes the input keying material and "extracts" from it a
fixed-length pseudorandom key K.
The second stage "expands" the key K
into several additional pseudorandom keys (the output of the KDF).
.Pp
.Fn EVP_PKEY_CTX_hkdf_mode
sets the mode for the HKDF operation.
There are three modes that are currently defined:
.Bl -tag -width Ds
.It Dv EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND
This is the default mode.
Calling
.Xr EVP_PKEY_derive 3
on an
.Vt EVP_PKEY_CTX
set up for HKDF will perform an extract followed by
an expand operation in one go.
The derived key returned will be the result after the expand operation.
The intermediate fixed-length pseudorandom key K is not returned.
.Pp
In this mode the digest, key, salt and info values must be set before a
key is derived or an error occurs.
.It Dv EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY
In this mode calling
.Xr EVP_PKEY_derive 3
will just perform the extract operation.
The value returned will be the intermediate fixed-length pseudorandom
key K.
.Pp
The digest, key and salt values must be set before a key is derived or
an error occurs.
.It Dv EVP_PKEY_HKDEF_MODE_EXPAND_ONLY
In this mode calling
.Xr EVP_PKEY_derive 3
will just perform the expand operation.
The input key should be set to the intermediate fixed-length
pseudorandom key K returned from a previous extract operation.
.Pp
The digest, key and info values must be set before a key is derived or
an error occurs.
.El
.Pp
.Fn EVP_PKEY_CTX_set_hkdf_md
sets the message digest associated with the HKDF.
.Pp
.Fn EVP_PKEY_CTX_set1_hkdf_salt
sets the salt to
.Fa saltlen
bytes of the buffer
.Fa salt .
Any existing value is replaced.
.Pp
.Fn EVP_PKEY_CTX_set1_hkdf_key
sets the key to
.Fa keylen
bytes of the buffer
.Fa key .
Any existing value is replaced.
.Pp
.Fn EVP_PKEY_CTX_add1_hkdf_info
sets the info value to
.Fa infolen
bytes of the buffer
.Fa info .
If a value is already set, it is appended to the existing value.
.Sh STRING CTRLS
HKDF also supports string based control operations via
.Xr EVP_PKEY_CTX_ctrl_str 3 .
The
.Fa type
parameter "md" uses the supplied
.Fa value
as the name of the digest algorithm to use.
The
.Fa type
parameter "mode" accepts "EXTRACT_AND_EXPAND", "EXTRACT_ONLY"
and "EXPAND_ONLY" as
.Fa value
to determine the mode to use.
The
.Fa type
parameters "salt", "key" and "info" use the supplied
.Fa value
parameter as a
seed, key, or info.
The names "hexsalt", "hexkey" and "hexinfo" are similar except they take
a hex string which is converted to binary.
.Sh NOTES
All these functions are implemented as macros.
.Pp
A context for HKDF can be obtained by calling:
.Bd -literal
 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
.Ed
.Pp
The total length of the info buffer cannot exceed 1024 bytes in length:
this should be more than enough for any normal use of HKDF.
.Pp
The output length of an HKDF expand operation is specified via the
length parameter to the
.Xr EVP_PKEY_derive 3
function.
Since the HKDF output length is variable, passing a
.Dv NULL
buffer as a means to obtain the requisite length is not meaningful with
HKDF in any mode that performs an expand operation.
Instead, the caller must allocate a buffer of the desired length, and
pass that buffer to
.Xr EVP_PKEY_derive 3
along with (a pointer initialized to) the desired length.
Passing a
.Dv NULL
buffer to obtain the length is allowed when using
.Dv EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY .
.Sh RETURN VALUES
All these functions return 1 for success and 0 or a negative value for
failure.
In particular a return value of -2 indicates the operation is not
supported by the public key algorithm.
.Sh EXAMPLES
This example derives 10 bytes using SHA-256 with the secret key
"secret", salt value "salt" and info value "label":
.Bd -literal
EVP_PKEY_CTX *pctx;
unsigned char out[10];
size_t outlen = sizeof(out);

if ((pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL)) == NULL)
	/* Error */

if (EVP_PKEY_derive_init(pctx) <= 0)
	/* Error */
if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
	/* Error */
if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
	/* Error */
if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
	/* Error */
if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
	/* Error */
if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
	/* Error */
.Ed
.Sh SEE ALSO
.Xr EVP_PKEY_CTX_ctrl_str 3 ,
.Xr EVP_PKEY_CTX_new 3 ,
.Xr EVP_PKEY_derive 3
.Sh STANDARDS
RFC 5869: HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
.Sh HISTORY
.Fn EVP_PKEY_CTX_set_hkdf_md ,
.Fn EVP_PKEY_CTX_set1_hkdf_salt ,
.Fn EVP_PKEY_CTX_set1_hkdf_key ,
and
.Fn EVP_PKEY_CTX_add1_hkdf_info
first appeared in OpenSSL 1.1.0 and
.Fn EVP_PKEY_CTX_hkdf_mode
in OpenSSL 1.1.1.
These functions have been available since
.Ox 7.2 .
